1. Field of the Invention
The present invention relates generally to fluorescence resonance energy transfer and protease assays and, more specifically, to cell-based methods for assaying for clostridial toxin activity.
2. Background Information
The neuroparalytic syndrome of tetanus and the rare but potentially fatal disease, botulism, are caused by neurotoxins produced by bacteria of the genus Clostridium. These clostridial neurotoxins are highly potent and specific poisons of neural cells, with the human lethal dose of the botulinum toxins on the order of nanograms. Thus, the presence of even minute levels of botulinum toxins in foodstuffs represents a public health hazard that must be avoided through rigorous testing.
However, in spite of their potentially deleterious effects, low controlled doses of botulinum neurotoxins have been successfully used as therapeutics and for some cosmetic applications. In particular, botulinum toxins have been used in the therapeutic management of a variety of focal and segmental dystonias, strabismus, and other conditions in which a reversible depression of a cholinergic nerve terminal activity is desired. Established therapeutic uses of botulinum neurotoxins in humans include, without limitation, blepharospasm, hemifacial spasm, laringeal dysphonia, focal hyperhidrosis, hypersalivation, oromandibular dystonia, cervical dystonia, torticollis, strabismus, limbs dystonia, occupational cramps and myokymia (Rossetto et al., Toxicon 39:27-41 (2001)). As an example, intramuscular injection of spastic tissue with small quantities of botulinum neurotoxin A has been used effectively to treat spasticity due to brain injury, spinal cord injury, stroke, multiple sclerosis and cerebral palsy. Additional possible clinical uses of clostridial neurotoxins currently are being investigated.
Given the potential danger associated with small quantities of botulinum toxins in foodstuffs and the need to prepare accurate pharmaceutical formulations, assays for botulinum neurotoxins presently are employed in the food and pharmaceutical industries. The food industry requires assays for the botulinum neurotoxins to validate new food packaging methods and to ensure food safety. The growing clinical use of the botulinum toxins necessitates accurate assays for botulinum neurotoxin activity for product formulation as well as quality control. In both industries, a mouse lethality test currently is the only acceptable assay for botulinum neurotoxin activity.
Unfortunately, the mouse lethality assay suffers from several drawbacks: cost due to the large numbers of laboratory animals required; lack of specificity; the potential for inaccuracy unless large animal groups are used; and the necessary sacrifice of animal life. Thus, there is a need for a new method that can complement and reduce the need for the mouse lethality assay. In addition to measuring toxin proteolytic activity, such a surrogate method also should require cellular uptake of the toxin and delivery of the toxin light chain into the cell cytosol. The present invention satisfies this need by providing novel cell-based assays for clostridial toxin activity and also provides related advantages.